Air turbine starters are conventionally employed in the aerospace field for starting of propulsion turbine engines of various types. For aircraft propulsion engine starting, the air turbine starter may receive pressurized air as motive fluid from a storage source aboard the aircraft, from another propulsion engine of the aircraft, or pressurized air may be provided by a portable ground start cart. Such carts usually employ a turbine engine or piston engine to drive an air compressor. Whatever source of pressurized air is employed, the air turbine starter accelerates the propulsion engine from a stop to and beyond its self-sustaining speed. Even after the propulsion turbine engine attains self-sustaining speed, torque delivery from the air turbine starter is continued to assist acceleration of the propulsion turbine engine toward its operating speed range. When the air turbine starter reaches its cut out speed, that is, the upper limit of its operating speed range, the supply of pressurized air to the starter is cut off. The propulsion turbine engine continues to accelerate toward its operating speed range, while a clutch prevents back driving of the air turbine starter by the turbine engine. During operation of the propulsion turbine engine an electric generator and hydraulic pump may be driven from the engine. These accessory devices are generally employed to power electrical and hydraulic devices of the aircraft.
On the other hand, it is desirable to operate various electrical and hydraulic systems of an aircraft while the latter is on the ground. This operation of the accessory devices is generally for the purpose of verifying correct operation, or performing maintenance procedures and checks, and is generally referred to as ground check out (GCO) of the aircraft. Unfortunately, operation of the propulsion turbine engine may be required in order to operate the accessory devices to provide ground check out power for the aircraft. Of course, operating a propulsion engine simply for ground check out purposes is undesirable. Such use adds operating time to the propulsion engine, is not fuel efficient, and presents a hazard to ground maintenance personnel around the aircraft.
However, operation of the propulsion engine may be necessary in order to drive the accessory devices within their normal operating speed range. Some success has been achieved at decoupling the propulsion engine from the accessory drive gearbox and driving the accessory devices via the air turbine starter while using a ground start cart to power the starter. This solution to the problem of providing ground check out power is not completely satisfactory because the starter must be operated above its starting speed range in order to drive the accessory devices fast enough. That is, the starter is being operated at a higher speed than it would experience during starting operation in order to power accessories which during their normal flight operation would be driven to a higher speed by the propulsion turbine engine. Such use of the air turbine starter, which may be protracted in comparison to its duty time in engine starting, both exposes the starter to long-duration high speed operation and resulting wear, and results in an inefficient power output from the starter. This latter effect results from the rapid drop off of torque production by a fixed-geometry air turbine once its speed of optimum torque production is exceeded. However this high speed operation of the starter is required to meet the speed range requirements of the accessory devices, and necessarily results in decreased operating efficiency during GCO.